/*
 * 	Author : Pallab
 * 	Prog: I.cxx
 *
 * 	Created on: 12:03:19 AM - 2013:2:22
 * "I have not failed, I have just found 10000 ways that won't work."
 */
#include <iostream>
#include <algorithm>
#include <vector>
#include <sstream>
#include <fstream>
#include <string>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <numeric>
#include <stack>
#include <functional>
#include <bitset>
#include <iomanip>

#include <ctime>
#include <cassert>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <climits>
#include <cstring>
#include <cstdlib>

using namespace std;


#define foR(i,frm,upto) for( int i=(frm),_i=(upto);(i) < (_i) ; ++i )
#define foD(i,frm,upto) for( int i=(frm),_i=(upto);(i) >= (_i) ; --i )
#define foit(i, x) for (typeof((x).begin()) i = (x).begin(); i != (x).end(); i++)
#define Int long long
#define pb push_back
#define SZ(X) (int)(X).size()
#define LN(X) (int)(X).length()
#define mk make_pair
#define dbg(...)       printf(__VA_ARGS__)
#define SET( ARRAY , VALUE ) memset( ARRAY , VALUE , sizeof(ARRAY) )
#define line puts("")

inline void wait ( double seconds ) {
    double endtime = clock() + ( seconds * CLOCKS_PER_SEC );
    while ( clock() < endtime ) {
        ;
    }
}
template<class T>
    inline T fastIn() {
    register char c=0;
    register T a=0;
    bool neg=false;
    while ( c<33 ) c=getchar();
    while ( c>33 ) {
        if ( c=='-' ) {
            neg=true;
        } else {
            a= ( a*10 ) + ( c-'0' );
        }
        c=getchar();
    }
    return neg?-a:a;
}
int const mxn = 20005;
char grid[mxn];
int N,K;
inline void read() {
    N=fastIn<int>(),K=fastIn<int>();
    scanf ( " %s",grid );
}
inline pair<int,int> getNext ( int at, char const &prv ) {
    int matchCount=0;
    while ( at<N && prv==grid[at] ) {
        ++at;
        ++matchCount;
    }
    return mk ( at,matchCount );
}
inline void proc() {
    int need=0;
    bool TRUE=false;
    for ( int i=0; i<N ; ) {
        pair<int,int> tmp = getNext ( i, grid[i] );
        //1: match count is greater jump = valid
        if ( tmp.second >= K ) TRUE|=true;
        need+= ( tmp.second/K ) + ( tmp.second%K?1:0 );
        i=tmp.first;
    }
    if ( TRUE ) cout<<need<<"\n";
    else {
        puts ( "-1" );
    }
}
int main() {
    int kase = 1;
#if defined( xerxes_pc )
    if ( !freopen ( "in1", "r", stdin ) )
        puts ( "error opening file in " ), assert ( 0 );
    kase = 1;
#endif
    kase=fastIn<int>();
    foR ( i,0,kase ) {
        read();
        proc();
    }
    return 0;
}
// kate: indent-mode cstyle; indent-width 4; replace-tabs on; 
